1. Field of the Invention
The present invention relates to methods and modems for transmitting digital data and more particularly to the method of generating signals having concentrated spectral occupancy of the channel bandwidth and coding gain and to voice channel modems utilizing such signals.
2. Description of the Prior Art
The wide use of digital signals often requires transmission of digital data over narrow band communication channels, such as voice grade telephone line. A typical usable bandwidth is therefore in the range of 700 Hz to 2700 Hz. Similarly, voice bandwidth channels transmitted over microwave and satellite links may be required to carry digital data. A number of well known techniques are commonly used for transmission of digital signals over restricted bandwidth channels. Typically, systems will utilize a carrier which may be modulated by on/off keying, by frequency shift keying or by phase shift keying. At the receiving end, a decision is made as to which level of a binary signal was transmitted. The detectability of the received signal is determined by relative magnitude of the difference signal: that is, the energy in the signal representing the difference between a ZERO and a ONE. Assuming that the energy per symbol is fixed at the transmitting end, increasing the rate of transmission will reduce the amplitude of the difference signals at the receiving end, resulting in a greater error rate in the presence of system noise. Thus, these simple one-coordinate type techniques are not suitable for transmission of high speed data in narrow channels.
When it is required to improve the detectability of the signal for a fixed transmission power, it is common to use a two-coordinate system. A commonly used technique utilizes orthogonal trigonometric coordinates to produce an 8-phase signal for transmission of 4800 bits per second over a nominal voice grade channel. These systems have a symbol rate of 1600 Hz to produce a 4800 bit rate or approximately 3 bits per Hz. In order to approximately center the spectrum of the transmitted signal, a carrier is generally chosen at about 1700 Hz. The signal set has an alphabet of 8 phases with each vector separated by 45.degree.. The transmitted signal is the linear sums of the two orthogonal components of the 1700 Hz carrier. The starting phase of the carrier may change at the beginning of every baud, and, consequently, the receiver must track the carrier to determine which signal was transmitted. There is no correlation of the signal from baud to baud and the system is not optimized for spectral occupancy. A low pass filter may be utilized at the transmitting end to filter out the higher frequency components and the line itself will also limit the spectrum. These elements tend to introduce intersymbol cross-talk and distortion which reduces detectability. Therefore, the demodulator at the receiving end must first extract the carrier from the incoming signal, it must obtain baud synchronization, it must equalize as far as possible the intersymbol interference, and finally, must make the bit decisions. It is also common to utilize transmission time for sending preambles and the like to initiate synchronization. Although the 8-phase system is transmitting digital information, it is, in essence, an analog system and is not easily implemented with logic circuits or in LSI form.
To obtain improved performance of modems for transmission of binary signals over narrow band communication channels, a signal design is required that will more accurately concentrate the spectral components of the transmitted signal into the band limits of the channel. Such a signal would require less equalization and would have reduced intersymbol interference. Next, the signal design should be such that it could be implemented digitally at the transmitting end so as to be easily implemented by LSI circuit elements. Therefore, a modem utilizing such a signal design would be capable of much better performance over telephone lines for high bit rate transmission. In addition, such a modem would have application to satellite channels in which the transmission power is limited and in which an improvement of a relatively small amount can result in a highly reliable communication channel.